Power line carrier frequency telephone system



June 13, 1950 J. M. BARs'row ETAL POWER LINE CARRIER 'FREQUENCY TELEPHONE SYSTEM 2 Sheets-Sheet 1 Filed OO'L. 11l 1946 J. M. BARSTOW A c. LA /vc ATTORNEY /N VEN TORS June 13, 1950 J. M. BARsTow :E1-Al.l 2,511,616

POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM@ Y Filed oct. 11, 194s 2 sheets-sheet 2 JM. ARsTow NVENTORS ,4.6. LANG @0. CMM

ATTORNEY Patented June 13, 1950 POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM John M. Barstow, Mountain Lakes, N. J., and Albert G. Lang, Whitestone, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N Y., a corporation of New York Application October 11, 1946, Serial No. 702,734

7 Claims.

This invention relates to communication sys- Y tems and particularly to telephone systems including subscriber stations arranged for carrier current operation over a power transmission line.

Objects of the invention are the provision of carrier telephone service over a power transmission line, the provision of means for automatically guarding the telephone line to the telephone central oiiice associated with the power line on calls between telephone stations connected to the power line, the elimination of manual control means at these stations heretofore provided for use only on such calls, and in general the simplification and improvement of power line carrier telephone systems.

This invention is a telephone system comprising subscriber stations arranged for carrier current operation over a power transmission line in which carrier current of one frequency is used for transmitting over the power line voice and signal currents to any one of these stations and carrier current of another frequency is used for transmitting over the power line voice and signal currents from any one of these stations, except that on reverting calls the carrier frequencies for transmitting voice and signal currents to and from the calling station are automatically interchanged. A feature of the invention is the provision of `means at each carrier station for automatically transmitting over the power line on a reverting call a reverting call signal consisting of carrier current of a third frequency. An understanding of the invention will be facilitated by considering a system in which the invention and its features are embodied, such a system being represented schematically in the drawing. The invention is, however, not limited in its application to the system shown but is generally applicable to telephone systems.

The drawing, which consists of two figures,

represents an automatic telephone exchange serving subscriber stations some of which are arranged for carrier operation over a power transmission line. Fig. 1 shows an automatic telephone exchange -comprising a subscriber station l0, subscriber 'lines Il and 2| and line circuits I2 and 22, linender switches LFI and LF2, selector switches Sl and S2, connector switches Cl and C2, a carrier terminal station or equipment unit CT individual to the line 2l, and a power transmission line PL to which the carrier terminal equipment VC'l' is coupled by coupling means 44.

Fig. 2 shows one of a plurality of subscriber stations connected to, andarranged for carrier current operation over, the power line PL.

The subscriber station l0 is of the usual type provided for use on lines terminating in an automatic central office and includes a dial or impulse sender (not shown) for use in selectively controlling the switches through which desired connections are established. The line-nder, selector and connector switches are of the two motion step-by-step type, each of the line-finder and selector switches being represented in the drawings by a set of brushes and a single set of terminals, and each of the connector switches being represented by a set of brushes and two sets of terminals. 'Ihe relays and other apparatus associated with the line-finder, selector and connector switches are represented by broken lines. Reference may be had to Automatic Telephony by Smith and Campbell, 2nd edition, pages 53 to 65 for a detailed description of the structure of twomotion step-by-step switches and their operation when used as selectors and connectors. Reference may be had to the patent to R. L. Stokely 1,799,654, granted April 7, 1931, for disclosure of a line circuit and aline finder of the step-by-step type. The ringing code generator 21 comprises a source of ringing current and means for inter.- rupting the ringing supply conductors as required to produce a plurality of different ringing codes for signaling subscriber stations. Reference may be had to the patent to Duguid No. 2,225,907, granted December 24, 1940, for disclosure of such la ringing code generator and to the patent to 'PL and the carrier telephone subscriber stations oi Fig. 2 constitute a power line carrier telephone system similar generally to the system disclosed in the copending application of R. C. Edson and J. W. Emling, Serial No. 653,254, filed March 9, 1946, now Patent No. 2,481,915, granted September 13, 1949, to which application reference may be had for discussion of the carrier frequencies suitable for use in such a system and for disclosure of the coupling means between the power line and the carrier receivers and transmitters and details of the carrier terminal equipment CT and carrier telephone stations. The power line PL is one phase of a power transmission line which serves a plurality of power and light customers, some of which are equipped for carrier telephone service as shown in Fig. 2.

The carrier terminal station or equipment unit -CT comprises a hybrid coil or group of transformer windings 30 arranged to transmit signal and voice currents to and from line 2|. The resistor 3| and condenser 32 constitute a network connected to the middle right pair of windings t balance the line 2|. The upper pair of windings receive voice frequency currents from the carrier receiver comprising amplifier-demodulator 50 and the lowest pair of windings transmit voice frequency currents from line 2| to the input circuit of the carrier transmitter comprising oscil. lator-modulator-amplifier 40. The power line PL is connected through coupling means 44 and conductors 43 to the carrier receiver comprising receiving band filter 49 andthe associated amplifier-demodulator 50, to thev carrier receiver comprising receiving band filter 59 and associated amplifier-demodulator 66, and to the carrier transmitter comprising transmitting band filter 4| and oscillator-modulator-amplifier 4|). The amplifier-demodulators 56 and 69 and the oscillatcr-modulator-amplifier 48 may be of any known suitable type. The oscillator-modulator 40 is tuned to transmit through filter 4 carrier current ofv a. particular frequency FI as modulated by either signaling or voice currents incoming over line 2|. The amplifier-demodulator 50 receives through filter 4.9 from the power line PL, carrier current of; another particular frequency F2., modulated by voice and signal currents from any one of the carrier telephone stations connect- .ed to the. power line PL.. The amplifier-demodulater 66 receives through filter 59 from power line PL, carrier current of frequency F3 which is transmitted from anyone of the carrier telephone stationsy while a reverting call is being made. therefrom. The. transmitting and receiving filters4| and.y 49. transmit the voice frequency sidebands in addition to the carrier frequency.

carrier terminal equipment CT further comprises relay 35 controlled by ringing current incoming over line 2| to connect battery B2 to oscittatot-modulator-amplier apparatus 40 and thereby effect the transmission over power line PL of impulses of carrier current corresponding to the ringing code received by relayl 35 from line 2|. The. carrier terminal equipment CT further comprises relays 5|. and 5,2. controlled by ampli- .er-demodulator 59. responsive to signals transmitted over the power line b y carrier current of frequencyv F2 f romV any of the carrier stations, and also comprises. a relay 52| controlled by amplifier-demodulator- 68 responsive to signals transmitted over the power lineA by carrierV current, of frequency F3. The functions of each of these relays is hereinafter described in detail.

Each of the carrier telephone subscriber stations is similar to the. one shown in Fig. 2 and is connected through.` individual coupling means 69 to thepower line; PL. Each station comprises a carrier receiver, a carrier transmitter, a telephone receiver 84. and transmittery 85. combined in a handset which normally rests. on the receiver hook 8.3', a. dial 8|A for controlling automatic switches in thetelephone central office. to establish desired connections, a. relay 16v and a. ringer 'L9- The carrier receiver comprises a receiving band filter 1| tuned to the carrier frequency F|, a receiving band filter 12 tuned to the carrier fre.- quency F2, and amplifier-demodulator apparatus 1:3. rI=`he carrier transmitter comprises oscillatormodulator-amplifier apparatus. 88; and a transmitting band filter 89 for transmitting voicernodulated carrier currentLof. either frequency lil` or frequency F2 depending upon whether the call is an incoming or an outgoing call, a frequency control relay 90, oscillator crystals. 9| and 92, andA a transmitter control relay 9.3. for connectingA the output conductors of filter 89 to the power line. The receiving and transmitting filters transmit the Voice frequency sidebands in addition to the carrier frequency. The amplifier demodulators and oscillator-modulator-amplifier may be of any known suitable type. Each station further comprises an oscillator-amplifier 94 and an associated transmitting band filter 95 tuned to transmit carrier current of frequency F3 over the power line as a reverting call signal on reverting calls. The power supply for the oscillator, modulator, demodulator andr amplifier apparatus includes a rectifier 61 and a transformer 66, connected to the power line either directly or through an ntermediate transformer. Other transformer windings may provide the energy for heating the filaments of vacuum tubes and supply whatever other operating potentials are required. No revertive call key is providedsince the transmission of a revertive call signal of carrier frequency F3 is automatically effected.

Call from station. 10 to a carrier telephone station Assume that a call is initiated at station 0, that the line is connected py a line finder LFI to a selector S| that the selector S| is selectively controlled by dial impulses from station I0 to select a group of trunks and an idle trunk therein associated with a connector CI, and that the connector C! is selectively controlled by. dial impulses from station |81 to selectA the line 2|-. Assume further that the line 2| is idle when tested by the connector C| that a ringing code is selected responsive to dial impulses from station |0, and that the selectedl ringingI code is transmitted through the brushes of connector C| over the conductors of line 2| through condenser 33, rectifier 34, and a back contact of relay 52 to the winding of relay 35. Relay 35 is thereby operatively energized during each ringing interval of the ringing code transmitted over line 2|. The operation ofv relay 35 connects the positive pole of battery B2 tol conductor 39 to` energize the oscillator-modulator-amplifier 40, disconnects the lower pair of windings of hybrid coil 30 from the input conductors of modulator 4U, and connects these conductors through condensers 36 to the conductors of line 2|.; whereby the ringing current incoming over line. 2.|' modulates the carrier currentV transmitted through filter 4|, conductors 43 and coupler 44 over the power line PL to each ofthe carrier telephone stations. connected to the power line.v

Ateach of the. carrier telephone subscriber stations the ringing current modulated'. carrier of frequency Fl' incoming, over power line PL is transmitted through-a coupler 69., conductors 10, and a normally closedl contact of the springs controlled by receiver hook 83, through receiving band filter totheinput. circuit of amplifier-demodulator 13. The output` circuit. ofamplierdemodulator 1.3. includes resistor 14 and the winding of relay 16; and relay 16 is operated responsive to carrier current` incoming. over the power line. A condenser 15 connected. in parallel with the winding of relay-T5, renders this relayzsomewhatv slow in operating: soas to tend to prevent its operation respnnsive to interfering.; currents such as static. The output conductorof modulator v'I3 is; furtherconnected throughcondenser 8| thencel through the olf-normal contactsof dial 86 and telephone. receiverail: in parallelwith re.- sistor 82. While rthe;telephonehandsetis in; normalY position on; hook 83 receiver' 84 f andv resistor 82Y are shortecircnitedat .a. haelt contact;y of the,

springs controlled by hook 83. The operationof relay 16 closes a circuit from the lower winding of transformer 66 through rectifier 61, conductor 68. resistor 80, front contact of relay 16, condenser 18, and through the windings of ringer 19 to ground at a back contact of the springs controlled by receiver hook 83. The windings of ringer 19 are thereby energized while condenser 1.8 is charging. .Since the incoming carrier current of frequency-FI is modulated by the frequency of the ringing current transmitted over line 2l from the central omce, relay 16 is alternately operated and released at the ringing current frequency (for instance 16 cycles per second) during each ringing interval of the ringing code. Each time relay 16 releases, condenser 18 discharges through resistor 11; and each time relay 16 reoperates, condenser 18 is charged through Yringer 19. Thus the ringer 19 at each of the carrier telephone subscriber stations associated with power line PL is actuated in accordance with the ringing code of the particular station which is being called.

When the handset is removed from the hook 83 at the called station, the circuit through ringer 19 is opened, the short circuit across the receiver 84 is opened, and the positive potential supply conductor 68 is connected to the winding of relay 93 and to the oscillator-modulator-amplier 88, operating relay 93 and energizing the ampliermodulator 88. Relay 93 is somewhat slow in operating to allow time for relay 90 to operate if relay 16 is operated at the time the handset is removed, as is the case at the calling station on a reverting call as hereinafter described. However, on the call being described, relay 16 is not operated at the called station when the handset is removed because at this time there is no carrier current of frequency F2 being transmitted over the power line, so that relay 98 is not operated; and the operation of relay 93 disconnects the winding of relay 90 from the armature of relay 16, thereby to prevent the operation of relay 90 on the call from station I8. Since relay 98 is normal, the oscillator crystal 92 is included in the oscillating circuit of oscillator-modulator 88 whereby carrier current of frequency F2 is generated and transmitted through filter 89, a front contact of relay 93, conductors 18, and coupler 69, over power line PL to each of the other carrier stations and to the carrier terminal equipment CT. The carrier current of frequency F2 does not reach the demodulator at the called station since relay 93 is operated and relay 99 is not operated; and does not reach the demodulator 13 at each of the other carrier subscriber stations because, at each of these stations, the input circuit of filter 12 is open as long as the handset is in normal position on hook 83. But at the carrier terminal .equipment CT, the carrier current of frequency F2 passes through coupler 44, conductors 43 and receiving band filter 49 to the input circuit of amplier-demodulator 58, causing the operative energization of the winding of relay l. The operation of relay 5| closes a circuit for operating slow-to-release relay 52 and closes a bridge consisting of the left middle pair of windings of hybrid coil 38, across line 2| to trip the ringing in the connector CI. The opening of the ringing circuit and closing of the talking connectionthrough connector CI is effect# ed in usual and well-known manner. The opera. tion of relay 52 opens the circuit vconnection between the winding of relayV 35 and line 2|, and connects Vthe positive pole of batteryBlto the ampliner-dem'dulator 60 and to the oscillator modulator-amplier 48. The energization of the amplifier-@demodulator A6l! performs no useful function on this call, but the energization of oscillator-modulator-amplifler 48 effects the transmission of carrier current of frequency Fl through filter 4 Ir over the power line PL to each ofthe carrier stations, causing the energization of relay 16 at each of these stations. The voice currents incoming over line 2l from the calling station I8 are transmitted through hybrid coil 38 to the input circuit of modulator 40 to modulate the carrier current of frequency FI trans-- mitted over the power line to the carrier stations.. At the called station, the voice current outputlof amplier-demodulator 13 is transmitted through the receiver 84, but at the other carrier stations,. the-receiver is short-circuited at a contact of the: receiver-hook springs. The transmitter of the handset of the called station generates voice currents which are transmitted through coil 81' to the input circuit of modulator 88, to modulate the carrier current of frequency F2 which is being transmitted over the power line. The voice modulated carrier current of frequency F2 passes through filter 49 to demodulator 50; and the voice currents are transmitted through hybrid coil 38 over line 2|A through the connector, selector and line-nder switches, over line Il to the calling station I8. Two-way conversation now takes place over the established connection including the power line PL between the calling and called station.

When the handset is replaced on hook 83 of the called station, receiver 84 is short-circuited, relay 93 is released, and the oscillator-modulatorampliiier 88 is deenergized, terminating the transmission of carrier current of frequency F2 over the power line and thereby causing the successive release of relay 5l and 52 of the carrier terminal equipment CT. The release of relay 5l opens the bridge across line 2| thereby releasing the answering supervisory relay (not shown) in the connector CI. The release of the relay 52 disconnects battery B2 from oscillator-modulatoramplier 48 to terminate the transmission of the carrier current of frequency Fl over the power line, thereby releasing relay 16 at each of the carrier stations. When the subscriber at the calling station I0 releases the connection, the con-v nector Ci, selector SI and line nder LFI are restored to normal 'in usual and well-known manner.

Call from 'carrier telephone station Assume next that the handset is removed from Y hook 83 at one of the carrier telephone stations connected to power line PL to initiate a call. Relay 93 is thereby operated, relay 90 remains normal, and the oscillator-modulator-amplier 8.8 is energized to transmit carrier current of frequency F2 over power line PL causing the successive operation of relays 5l and 52 of carrier terminal equipment CT. At each of the other carrier subscriber stations, the carrier current of frequency F2 transmitted over line PL from the callingY carrier station is ineffective to operate relay 16 because the input conductors 18 are normally connected across the input conductors of filter 'Il and are not normally connected across the input conductors of lter 12. The operation of relay: 5| of carrier equipment CT closes the bridge across line 2i whereby the line relay (not shown) of 'line circuit22 is operated to start an idle line finder .such as LF2 to connect with the calling line; .andline 2| is thus connected through the brushes of linerv finder Ll2-r to. the associated selector S2. The operationk of relay 52 opens the circuit connection between relay and'. line. 2| and connects. battery B2 to oscillator-modulatoramplier and to amplier-demodulator 6.0. The energization of oscillator-modulator-ampli er 40. effects the transmission of carrier current offrequency FI over power line PL. Dialtone is transmitted' from selector S2 over linev 2| throughhybrid: coil 3.8 to the inputzofA modulator Ml. to modulate the carrier current of frequency FI being transmitted over the power line. to. the carrier telephone station. The carrier currentof frequency FI effects the operation of relayV 16 at each.v of. the carrier stations; and,.a.t the. calling carrier station, the dial tone is` transmitted through the receiver 84'- to indicate that the dial.- ing 0f.- the. number of. the telephoneY station with which connection is desiredmay. be started. The calling. party thereupon dials. thedirectoryV num.- ber ofthe called station. When the dial is moved ofenormal, the receiver 8 4 is disconnected from amplifier-demodulatorl. and,.as the dial returns to normal, the impulse contacts of the dial modulate the carrier current of frequency F2 being transmitted from the oscillator-modulator-ampliiery 88.A Each dial impulse thus transmitted from the*v callingv station over the power line causes. the release and reoperation of relay 5|. Each releaseandreoperationof relay 5| respon-v sive to dial impulses, opens and closes the-bridge across line 2|l thereby repeating the dial impulses to selectively control the operation', of selector. S2fand a1 connector C2 to extend theconnection tothe calledline and to selectthe ringing code required for signaling. theA called station, Sincerelay 5,2V isslow. to release, it remains^ oper.- ated; duringt the. response of. 1re/.lay-y 5 la to. dial, im.- pulses` Assume rst that the 4 calledY station is not Vone of` theA carrier, stations connected; to the power linelL.V Whenthe c allis ansyverved thel connector C2; opens. the ringing circuitl and, closes the talking circuit.A in usual andnl well-.known manner. Talking. currents are transmitted through. hybrid; coil 38 and, over` thepowerline in both-.directions. Thenconnection isunderthe control ofl the; calling carrier. telephone` station; and; when the handset ,of thisfstationis` restored to normal condition onthehook 83, relay 93, is releasedgandthe. amplicrfmodulator 8.8 is. deenergized terminating the,`4 transmission A-of carrier current of frequency F2 and thereby cans: ing the successive release of relays 5|. and 52 of the carrier terminal equipment CT. The release of relay 5| opens the bridge across line 2| and the release of relay 52deenergizes the oscil. lator-rrmodulator-ampliiierV 48. Theopening of the bridge across line 2| causes the release of the line relay. (not shown) of the-connector,AV C2 and return of the selector S2 andline nder LEZto. normal in usual manner. Theconnector C2 is also restored to .normal in usual manner; assoon as both the calling and called stationshave released the connection.r

Assume next that the call initiated at acarrier telephone subscriber station asabove described: is a call to, another subscriber station connected-to the power line PL.` In such-vacasef, the connector C2A nds the` calledline 2| busy when it makes the busy test; and a-busy-tone is thereupon transmittedr from the connector, through selector S2,A andilin'e nder LFZ, over -line 2|, through hybrid coil.30; modulatorfamplier .48, lter 4|, power line PL; filter 1|; amplifier.

demodulator 13, and. condenser 8|-, to telephone receiver 84' at the callingstation. The connector C2' is arranged. to make a reverting call test, and, if the calli is a reverting call', the line' finder LF2 and selector S2 are restored to normal when the calling. subscriber hangs. up; but the connector C2 remains. connected to the line 2| to ring. the called station. Reference may be had to the patentto H. Hovland 1,849,087, granted March 15, 1932-, for disclosure of a connector arranged to. makethe aforementioned reverting call. test. Upon hearing. the busy tonev transmitted from connector` C2.; the` calling subscriber hangs the handset on the hook 83. Relay 93 releases and the oscillator-modulator-amplifler 88 is deener'- gizedto terminate the. transmission of carrier currentoffrequencyFZ over they power line and thus release relay 5| of the carrier terminal equipment CT.. The release of relay' 5| opens the bridge across the conductors of line; 2|, the line finder LFZ and selectorv S2 are restored to normal, and the connector C2 is operated to close the ringing circuit and apply the ringing code of; the called` station to the conductors of line 2|. Therelease of. relay 5| also causes the release of relayv 52.; but. the energization of the ringing relay 3,5, responsive to ringing current received over line 2 If, electstlie energization of the oscillatormodulator-amplifier 40' to transmit ringing current.modulated carrier cur-rent: of frequency FI over the power line PL. At each of the carrier telephone subscriber stations, the. relay 16 responds tofthevringing current and causes actuationofthe-ringer '|9:.according to the ringing code offthe called'staton. When the calledsubscriber answerslby removing the handset from hook 83, relaySSfs operated and the oscillator-modulatoramplier-88 is energized to transmit carrier current over the power. line. Since relay 16 is not operated at thistime, because there is no carrier currentof frequency E2 being transmitted over the power line, relay 98. does not operate, and: with i relay f 9 El 1 normal,- oscllator crystalY 92 is connectedin the oscillator circuit of oscillatormodulator88so thaticarrier-current of frequency F2is transmitt'edtherefrom over power line PL. The transmission of carrier current of frequency F2. over. the power linePL from the called station effectsy the reoperationof relays 5| and 52 ofJ the' carrierterminal equipment CT. The operation offrelayl5|` closes thebridge across the conductors. of line 2| to trip ringing andfto operate. the answering supervisory-relay (not shown) of; thefconnector C2 inusual manner. The operationof'relay 52 connects the positive pole of battery. B2 to energize the oscillator-modulatoramplifier. 48; and also to energizeV the amplifierdemodulatorf: The energization ofthe oscillator-modulatorramplier 40 effects the transmissionofgcarrier; current-.of frequency' Fl over thepowerfline to energize relay 'I6l at eachA of thecarrier telephone stations.

Whengthe operationof-the'ringerat the calling carrier telephone -stationceases due tcvanswer of thecalliat'the-calledstation, the calling party removes; the handset from'y receiverA hook 83, thereby: opening theh connection between input conductors 10F-andthe input conductors of lter 'H'Y andconnectin-ginput? conductors I0` to the inputtconduotors fof-"lter, 12; Since, at this time, carrierfcurrent: of frequency-F2. is being transmitted over power.A line PL from the. called station; relaylfisfoperated at the calling station and removal of the'. receiverV closes a circuit tlnouglr.thef.winding of; relay; 9D, whichl circuit includes the outer right back contact of relay 93, a normally open contact of the springs controlled by hook 83, front contact of relay 1B, resistor 8|), conductor 68, rectifier 61 and the lower winding of transformer 56. Relay 99 is thus operatively energized before the slow to operate relay 93 operates; and relay 90 locks independent of relay 93. The operation of relay 90 connects the oscillator crystal 9| in the oscillating circuit of oscillator-modulator 88 in place of the crystal 92 whereby carrier current of frequency FI, modulated by voice current from transmitter 85, is transmitted from the calling station. With both of relays 90 and 93 operated at the calling station, the input circuit of filter F2 is closed and the input circuit of lter 1| is open; so that voice modulated carrier current of frequency F2 incoming from the called station is transmitted to the receiver 84. The aforementioned operation of relay 90 also connects the positive potential supply conductor 68 to oscillator-amplifier 94, to effect the transmission of carrier current of frequency F3 through transmitting filter 95 over the power line, through receiving filter 59 to amplifier-demodulator 60 thereby to effect the operation of relay 6|. The operation of relay 6| disconnects the positive pole of battery B2 from oscillator-modulator-amplier 49 to terminate the transmission of carrier current of frequency Fl from the carrier terminal equipment CT during the conversation between the calling and called subscribers station.

If the calling subscriber hangs up first, relay 93 at this station releases and oscillator-modulator-amplifier 88 and oscillator-amplifier 94 are deenergized, whereby the transmission of carrier current of frequencies FI and F3'from the calling station is terminated-causing the release of relay 6| of the carrier terminal equipment CT and the release of relay 1S at each of the carrier stations not involved in the call described. The release of relay 6| connects battery B2 to oscillator-modulator-amplier 49 whereby carrier current of frequency FI is now transmitted from the carrier terminal equipment CT over the power line causing the reoperation of relay 16 at each of the carrier stations. Relay is held operated by the carrier current of frequency F2 being transmitted over the power line from the called subscriber station. When the called subscriber disconnects, the oscillator-modulatoramplier 88 thereat is deenergized terminating the transmission ofcarrier current of frequency F2 over the power line, releasing relays 5| and 52 of the carrier terminal equipment CT. The release of relay 5| opens the bridge across the conductors of line 2| thereby causing the release of the connector C2; and the release of relay 52 again connects the winding of relay 35 across line 2| for response to ringing current on a succeeding call.

If the called subscriber hangs up first, the transmission of carrier current of frequencyF2 therefrom is terminated causing the release of relay 5| of the carrier terminal equipment CT and causing the release of relays 16 and 90 at the calling subscribers station. The release of relay 90 at the calling station effects the deenergization of oscillator-amplifier 94 and reconnects the oscillating crystal 92 in the oscillating circuit of oscillator-modulator 88 in place of the crystal 9|, whereby the transmission of carrier current of frequency FI is terminated and' the transmission of carrier current of frequencyV F2 is initiated at the calling subscribers station.,

Relay''lof the carrier terminal equipmnt'CT is thus reoperated before the slow-to-release relay 52 has released so as to hold the central office connection until both of the subscribers have disconnected. When the calling subscriber hangs up, relay 93 releases and the oscillator-modulator-amplifier 88 is deenergized at this station.

If the called subscriber fails to answer on a reverting call, the ringer at each of the carrier stations is actuated by the ringing current until the receiver is removed from the switehhook at some one of the stations. When the receiver is replaced at this subscriber station, the carrier terminal equipment CT is restored to normal as well as the' carrier equipment at the answering station.

What is claimed is:

1. In a telephone system comprising subscriber lines and switching means for connecting calling and called lines, a power transmission line, a plurality of telephone subscriber stations connected to said power line, and means comprising carrier terminal equipment connecting said power line to one of said subscriber lines, each of said stations comprising carrier current transmitting and receiving means for transmitting and receiving over the power line Voice and signal modulated carrier currents of either one of twoV frequencies and means for transmitting at the same time that carrier current of a first one of said two frequencies is being transmitted a particular signal consisting of carrier current of a third frequency, said carrier terminal equipment comprising carrier current transmitting means for ytransmitting over said power line to any of said stations carrier current of said first one of said two frequencies and for modulatingsaid carrier current with voice and signal currents incoming over said one subscriber line, carrier receiving means for receiving over said power` line from any of said stations voice and signal modulated carrier current of the other of said frequencies and for transmitting said voice and signal currents over said one telephone line, and means controlled by the carrier receiving means responsive to said particular signal for preventing the transmission of carrier current over the power line fromsaid carrier terminal equipment.

2. In a telephone system, a telephone line, a power line, a carrier terminal station interconnecting said lines, a plurality of telephone subscriber stations connected to said power line, each of said subscriber stations comprising a first carrier current transmitter arranged to transmit carrier current of either one of two frequencies and a carrier current receiver arranged to receive carrier current of the other of the two frequencies., a second carrier current transmitter at each subscriber station arranged to transmit over said power line to said carrier terminal station a signal consisting of carrier current of a third frequency, means at said carrier terminal sta. tion for transmitting over the power line car-J, rier current of a particular one of said two.fre quencies, and means at said carrier terminal station effective responsive to said signal to pre- Vent the transmission of carrier current of saidI one frequency over the power line from the carrier transmitting means at said carrier terminalstation. 3. In a telephone system, a telephone line, a power line, a carrier terminal station interconnecting said lines, a plurality of telephone subscriber lstations connected to said power line,

each of said subscriber stations comprising car.- rier transmitting means arranged to transmit carrier .current of .either one of two frequencies and carrier receiving means arranged .to lreceive carrier current of the other of the two frequencies, means .at each subscriber station for interchanging the transmitting and lreceiving carrier frequencies responsive .to energization of the receiver by carrier current of one of said two frequencies, means at each subscriber station for Atransmitting over said power line to said carrier terminal .station a signal consisting of carrier current of a third frequency simultaneously with the transmission of carrier current of the other of said two frequencies, carrier transmitting means at said carrier terminal station for transmitting over the power line carrier current of said other of said two frequencies, carrier receiving means at said carrier terminal station and means controlledby said carrier receiving means responsive to `said signal for preventing the transmission of carrier current of said other frequency from said carrier terminai station.

Ll. In a telephone system, a telephone line, a power line, a carrier terminal station interconnecting said lines, a plurality of telephone subscriber stations connected to said power line, said carrier terminal station comprising means for transmitting carrier current of a first frequency over said power line and for modulating said carrier current by voice and signal currents incoming over said telephone line, means for `receiving carrier currents of a second frequency incoming over said power line from any of said subscriber stations and for transmitting over said telephone linevoice and signal currents by which said carrier current is modulated, means responsive to carrier current of a third frequency incoming over the power line from any of said subscriber stations for preventing the transmission .0f carrier lcurrent over the power line from said carrier terminal station, means at each of said subscriber stations rfor receiving carrier current of leither of said rst and second frequencies and for transmitting carrier current of the other of said first and second frequencies, means at each of said subscriber stations for modulating .said transmitted carrier current with voice or signal currents, and means .at each of said subscriber stations for transmitting a signal consisting of current of said third frequency simultaneously with the transmission of carrier `current of said rst frequency.

5. In a .telephone system, a telephone line, a power line, a carrier terminal station interconnecting said lines, a plurality of telephone subscriber stations connected to said power line, said carrier terminal station comprising means for transmitting carrier current of a rst frequency over said power line and for modulating said carrier current by voice and signal current incoming over said telephone line, `means for receiving carrier current of a second frequency incoming over said power line from any of said subscriber stations and for transmitting over said telephone line voice and signal currents by which said carrier current is modulated, and means controlled by said carrier current receiving means responsive to carrier current of a third frequency forpreventing the transmission of carrier current of said rst frequency over the power line from the carrier transmitting means of said carrier terminal station, means at each of said subscriber stations for receiving carrier currents of either of said first and second frequencies and for transmitting .carrier current of the other of said first and second frequencies, means at each of said subscriber stations for modulating the transmitted carrier currents with voice or signal currents, and means at each of said subscriber stations rendered effective in response to carrier current of said second frequency for transmitting a, signal .consisting of current of said third frequency.

6. 'In a telephone system, a telephone line, a power line, a carrier terminal station interconnecting said lines, a plurality of telephone subscriber stations connected to said power line, each of said subscriber stations comprising carrier transmitting means arranged to transmit voice frequency modulated carrier current of either one of two frequencies and carrier receiving means arranged to receive voice frequency modulated carrier current of the other of the two frequencies, means at each subscriber station for transmitting over said 2power line to said carrier terminal station a signal consisting of carrier current of a third frequency simultaneously with the transmission of carrier current of a particular one of said two frequencies, means at said Ycarrier terminal station for transmitting over the power line carrier current of a particular one of said two frequencies, and means at said carrier terminal station effective responsive to -said signal to prevent the transmission of car--l rier current of said one frequency over the power line from the carrier transmitting means at said carrier terminal station.

7. 'In a telephone system comprising a telephone line, a power line, a carrier terminal station interconnecting said lines, and a plurality of telephone subscriber stations connected to said power line, each subscriber station comprising a telephone receiver and a receiver hook on which the receiver rests when not in use, means at said terminal station for transmitting carrier current of a first frequency over said power line and for modulating -said carrier current by voice and signal currents incoming over said telephone line, a first carrier current receiver at said terminal station tuned to receive voice modulated carrier current of a second frequency incoming over said power line and to transmit over said telephone line the voice frequency components of said incoming carrier currents, means responsive to signal current incoming over said telephone line and to carrier current of said second frequency incoming over said power line for energizing said means for transmitting carrier current of said first frequency, a second carrier current receiver at said terminal station tuned to receive carrier current of -a third frequency incoming over said power line, means responsive to carrier current of said third frequency incoming to said second carrier `current receiver for deenergizing said means for transmitting carrier current of said first frequency, a carrier current transmitter at each of said subscriber stations for transmitting carrier current of said second frequency on calls incoming from or extended over said telephone line and for transmitting carrier current of said first frequency on calls to another of said subscriber stations, a carrier current receiver at each of said subscriber stations normally tuned to receive carrier current of said first frequency, means at each of said subscriber stations responsive to removal of the receiver from the receiver hook for tuning said carrier current receiver to receive carrier current of said second frequency, a second carrier current transmitter at each of 13 said subscriber stations for transmitting carrier current of a, third frequency over said power line, and mea-ns for energizing said second carrier transmitter responsive to carrier current of said second frequency incoming over the power line at the time the receiver is removed from the receiver hook.

J OHN M. BARSTOW.

ALBERT G. LANG.

REFERENCES CITED The following references are of record 1n the file of this patent:

14 UNITED STATES PATENTS Number Name Date Clark Aug. 19, 1930 Sandalls, Jr. July 7, 1942 Honaman Sept. 8, 1942 Bowers Aug. 28, 1945 Haberl Oct. 16, 1945 Lang Nov. 11, 1947 

